1. Field of the Invention
This invention relates to a method for mounting an electronic component and, more particularly, to a method for accurately mounting electronic components required to be mounted with high mounting accuracy such as a quad flat package (QFP) having leads extending in four directions.
2. Description of the Prior Art
There is known an electrode component mounting apparatus having a carrier turntable which is provided with a number of suction nozzles for electronic components positioned in a circumferential direction thereof (e.g., see FIG. 4 of Japanese Patent Application Laid-Open No. 60-171000). The electronic component mounting apparatus of this type picks up electronic components provided by an electronic component delivery unit by indexing rotational movements of the carrier turntable, and feeds and mounts the electronic components on a printed circuit board positioned on an X-axis table and a Y-axis table. This type of electronic component mounting apparatus is widely employed as a high speed mounting apparatus since it can mount three or more electronic components per second on a printed circuit board.
The position of the electronic component picked up by the suction nozzle may deviate from the desired position in X, Y and .THETA. directions. In order to mount the electronic component on the printed circuit board, this positional deviation must be corrected. In a conventional electronic component mounting apparatus, a recognition unit such as a CCD camera constituting part of a positional deviation correcting means is provided along the path of the electronic component as it is moved by the indexing rotation of the carrier turntable. The positional deviations of the electronic component in the directions X, Y and .THETA. are detected by the recognition unit. The positional deviation in the direction .THETA. (rotational direction) is corrected by rotating the suction nozzle in the direction .THETA. around its own axis, and the positional deviations in the directions X and y are corrected by driving the X-axis table and the Y-axis table to move the printed circuit board in the directions X and Y.
Recently, electronic components having leads such as QFP have been mounted on a printed circuit board. The electronic components having such leads are difficult to mount on the printed circuit board by the above-described carrier turntable type mounting apparatus for reasons described below.
(i) Recently, electronic components have been required for high density and high integration devices. This has resulted in an increase in the number of leads for such components, up to 200 or more. Therefore, the lateral width and pitch of the leads are extrafine (e.g., 0.5 mm or less). Thus, precise mounting accuracy on this order is required and depending upon the camera used as the recognition unit, it is extremely difficult to precisely observe the leads so as to achieve the required accuracy.
(ii) Electronic components having leads have become large in size, up to 5 cm or more on one side thereof. In order that the large-sized electronic component be within the visual field of the camera, the magnification of the camera must be reduced, but if the magnification is reduced, its observing accuracy is inevitably lowered.
(iii) The positional deviation in the .THETA. direction has been connected by rotating the nozzle through the angle .THETA.. However, since it is very difficult to precisely rotate the nozzle by means of a motor, the positional deviation in the .THETA. direction is very difficult to correct accurately.
For the reasons described above, capacitor chips, resistor chips, etc. requiring relatively low mounting accuracy can be mounted on a printed circuit board at high speed by the above described carrier turntable type mounting apparatus. However, electronic components having a large number of leads cannot be mounted on a printed circuit board by the carrier turntable type mounting apparatus, but must be separately mounted at a low speed by a single head type mounting apparatus as shown in FIG. 3 of Japanese Patent Application Laid-Open No. 60-171000.
Since an electronic component having a large number of leads, such as a QFP, must be mounted with high mounting accuracy, the electronic component must be mounted on the printed circuit board, separately from the capacitor chips, resistor chips, etc. Accordingly the working efficiency is reduced. Since two types of mounting apparatuses, the carrier turntable type and the single head type, are provided and must be operated separately, facility costs are increased, and production management and maintenance management also require a great deal of labor.